The present invention relates to a numerical control device for transfer machines adapted to control transfer machines mounted on a transfer line. More particularly, the present invention is directed to a numerical control device for transfer machines having a zone signal output function.
Numerical control devices for transfer machines mounted on a transfer line are used as control devices for working machines, and assembling and inspecting machines, etc., for mass production lines in, for example, the automotive industry.
These numerical control devices require a sequence control function and an axis control function. Among these functions is a zone signal output function which is indispensable for the execution of the sequence control function, such as a synchronization between the machines which comprise the transfer line and a program selection at the restart of machining.
The zone signal output function will now be described with reference to the drawings.
FIG. 2 is a diagram illustrating a prior art numerical control device (CNC) 1 which is used to control a machine tool and the rotation of a servomotor 2. The servomotor 2 rotates in response to a command from the numerical control device 1, and a ball screw 3 is connected directly to the servomotor 2 and rotates in synchronism with the rotation of the servomotor 2. Thus, as the servomotor 2 is rotated, a tool post 4 is moved along the ball screw 3. The tool post 4 is fitted with a tool 4a, by which a workpiece 5 is machined into a desired shape. After the machining is finished, the workpiece 5 is moved along the transfer line to a position at which the next machining process will be executed.
During the machining of this workpiece which moved along the transfer line, it is necessary to have precise knowledge of the position (absolute axis position) of the tool post 4 and to control the sequence of the machining processes accordingly.
Conventionally, therefore, an overall stroke 6 for the movement of the tool post 4 is, divided into N (integer) number of equal parts. Zone numbers 1, 2, 3, . . . N are affixed individually to the N number of equal zones (N is 128 or 256). The numerical control device 1 detects the zone where the tool post 4 exists by a comparison made on the basis of current position data, to thereby determine the zone number concerned and output that number as a zone signal. A programmable machine controller) incorporated with the numerical control device reads the zone signal through a window, determines the next process in the sequence to be controlled, and performs an axis control.
When the overall stroke are 1 m and N is equal to 128, respectively. According to a conventional system, the zone detection unit is about 7.8 mm. That is, the detection unit is very imprecise, and thus it is difficult to set a machining origin for minimized machining cycles, or to optimize the start-stop timing for a loader and peripheral equipment. Also, this problem constitutes a restriction of the machine design. Even if a certain zone number is decided, the detection unit is always subject to errors in a range specified by this zone number, and thus the zone cannot be perceived within a range smaller than that of the detection unit.
The zone can be recognized with greater accuracy by increasing the number N to increase the number of points in the zone, and making the detection unit smaller. In this case, however, the load on a CPU of the numerical control device becomes greater, and thus the capacity of the axis control function is reduced.